FIG. 12 shows a first example of conventional inverter circuits. This example is disclosed in Japanese Patent Unexamined Publication No. H09-140155. Transistor module (TRM) 3 drives motor 4 following PWM signal 2 supplied from controller 1. Temperature transducer 5 is disposed near TRM 3.
As shown in FIG. 13, microprocessor 6 in controller 1 receives data from temperature transducer 5, and controls TRM 3 for preventing thermal breakdown.
FIG. 14 illustrates control over TRM 3.                Step 10001: start        Step 10002: When a rate of temperature rise (a temperature rise per unit time; ° C./second) detected by temperature transducer 5 exceeds a given value, a carrier frequency is lowered, thereby decreasing switching loss of TRM 3 for suppressing a temperature rise of TRM 3. When the rate of temperature rise lowers to not higher than the given value, the carrier frequency is restored.        Step 10003: If the rate of temperature rise does not lower to not higher than the given value although the carrier frequency is lowered, an operating frequency of motor 4 is lowered, so that the temperature rise of TRM 3 is suppressed.        
FIG. 15 shows a second example of conventional inverter circuits. This example is disclosed in Japanese Patent Unexamined Publication No. H05-227793. PWM control circuit 12 compares carrier signal CA generated by carrier generator 11 with a control signal. Switching elements 15 are PWM-controlled in response to the comparison, thereby driving motor 13.
Since driving at a higher carrier frequency for reducing magnetic noises increases the heat generated by switching elements 15, appropriate selection of the carrier frequency is required so that the temperature of switching elements 15 can fall within a rated one. In addition to this preparation, a load applied to driving motor 13 is monitored with a current supplied from PWM control circuit 12, and when overload detector 14 detects an overload, the carrier frequency is lowered. In other words, as shown in FIG. 16, comparator 16 compares signal DE indicating a load status with reference signal S00, and when the comparison shows DE>S00, it is determined that the motor is overloaded and switch SW is turned on. A difference between signal DE and reference signal S00 supplied from subtracter AD1 is fed into filter 17 via switch SW. Time constant T1 of filter 17 is set to be T1>T2. T2 is an acceleration or deceleration time of driving motor 13. Adder-subtracter AD2 finds difference S2 between output S1 from filter 17 and predetermined set value S01 indicating a rated load status. The carrier frequency is lowered in response to difference S2.
However, in the first conventional example, since the inverter circuit monitors a rate of temperature rise of the switching elements, a long continuous driving will sometime overheat the switching elements even if the rate of temperature rise is low. In the second conventional example, since only a load applied to the motor is monitored, if a temperature around the switching elements is high, the switching elements will be sometime overheated although the motor is not overloaded.